Allow installation on Laravel 9 projects

[imjoehaines]

Goal

https://github.com/bugsnag/bugsnag-laravel/pull/470 ended up with a conflict, so I've rebased it in this PR. I've also:

• added a changelog entry
• removed the temporary workaround to run the MR tests on Laravel 9
• added Laravel 9 to the unit test matrix
• bumped the bugsnag/bugsnag dependency to 3.27.0 (latest)

[imjoehaines]

is there anything we should be pruning from the unit test matrix?

I think it's ok — even though it's a huge matrix it finishes in ~1-2 minutes, which is faster than the license audit and Maze Runner tests